Electrolytic reduction of halogenated halomethylpyridine

ABSTRACT

HALOGENS ARE SELECTIVELY REPLACED WITH HYDROGEN IN HALOGENATED HALOMETHYLPYRIDINES BY ELECTROLYTIC REDUCTION.

United States Patent 3,687,827 ELECTROLYTIC REDUCTION OF HALOGENATED HALOMETHYLPYRIDINE James N. Seiber, Davis, Calif, assignor to The Dow Chemical Company, Midland, Mich. No Drawing. Filed Jan. 25, 1971, Ser. No. 109,635 Int. Cl. C07b 29/06; C07d 31/26 US. Cl. 204-73 R Claims ABSTRACT OF THE DISCLOSURE Halogens are selectively replaced with hydrogen in halogenated halomethylpyridines by electrolytic reduction.

BACKGROUND OF THE INVENTION Nagao et al. in US. 3,425,919 show the electrolytic dechlorination of chlorinated aliphatic compounds. It is also known that halogens can be removed from an aromatic nucleus by electrolysis. Thus, when a compound having both aliphatic and aromatic halogens is electrolyzed, it cannot be predicted whether the halogens would be removed specifically, nor could it be predicted from the art which halogens presumably subject to reduction would be reduced.

SUMMARY OF THE INVENTION It has now been found according to the present invention that a halogenated halomethylpyridine is specifically reduced in electrolysis reactions to remove the halogens from the methyl groups prior to reducing the halogens on the pyridine nucleus. Thus, by a simple electrolysis, the halogens on the methyl group of a halogenated halomethylpyridine are selectively removed.

The halogenated halomethylpyridines may be any of those of the formula cx ne,

X. G Y...

wherein X is F or Cl p is an integer of l-3 n is an integer of l-4 Y is CN, NR or OR where R is H or an alkyl of 1 to 6 carbons,

m is an integer of 0-2.

Preferred halopyridines contain only F or only Cl, i.e., where m=0 and X is F or Cl. Of special interest are those pyridines having only chlorine substituents, i.e., where each X is Cl, perhalogenated methylpyridines, i.e., wherein n+p=7, and the rnonoamino substituted halomethylpyridines, i.e., wherein Y is NHg and m=1. Also of special significance in the invention is the reduction of halomethylpyridines having the halomethyl group in the 2 or the 4 position.

The desired electrolytic reduction of the invention is carried out by techniques that are generally known. These techniques are described below and exemplified in the Specific Embodiments. Broadly, the starting halogenated pyridine is dissolved in a suitable solvent containing an electrolyte, the solution is added to an electrolysis cell and current is passed through the cell until the desired degree of reduction is obtained.

The concentration of the reactants in the electrolytic cell may vary widely as different reactants and solvents are employed in the reaction. As a general rule, reactant concentrations near saturation in the solvent are preferred.

3,687,827 Patented Aug. 29, 1972 "ice The design of the electrolysis cell used in the present invention is not critical. Numerous electrolytic cells known in the art may be readily employed in the present invention. Preferred electrolytic cells have cathodes of mercury or lead. The anode may be essentially any chemically inert material with graphite and platinum being especially preferred. Such preferred cell may be arranged in any conventional design, including a cell divider when a substituent susceptible to anodic oxidation, such as NH is present.

The electrolyte used in the present invention may vary widely. Preferred electrolytes in the invention are neutral or acidic salts. The use of salts of strong bases may be detrimental to the progress of the reaction because of the tendency of such electrolytes to enter into reaction with the halogens. Specific examples of preferred electrolytes include sodium p-toluenesulfonate, sodium acetate, ammonium p-toluenesulfonate, ammonium chloride, ammonium fluoride, tetramethylammonium chloride, and hydrochloric acid, sulfuric acid, acetic acid or phosphoric acid used alone or in combination with ammonia or a tertiary amine. Especially preferred is the use of ammonium acetate, ammonium halide where the anion is the same as the halogen reduced, or H 50 as the electrolyte. The concentration of the electrolyte may vary widely as different reactant concentrations, electrolytes, current densities and cathode potentials are employed.

The solvent employed in the electrolysis solution may vary Widely as dilferent reactants are employed in the electrolytic dehalogenation. The solvent should dissolve all or most of the starting material and the electrolyte and should be inert or at least not detrimentally reactive under the electrolysis conditions. Solvents preferred in the present invention include the lower alcohols, lower alkylene glycol monoalkyl ethers and dialkyl ethers and lower amides. Representative examples of these preferred solvents include: alcohols such as methanol, ethanol, isopropanol and isobutyl alcohol; lower alkylene glycol monoalkyl ethers and dialkyl ethers such as 2-methoxypropanol, ethoxyethanol, dimethoxyethane and 1,2-dimethoxypropane; and lower amides such as dimethylformamide and acetamide. These solvents of the present invention may be used either alone or in combinations to give a conductive medium. Preferred cell fluids have up to about 30% by weight of water to assure proper solubility of the electrolyte.

In the operation of the electrolysis cell, the cathode potential is usually greater (in terms of absolute number) than about -1 volt but not so high that the electrolysis medium or electrolyte is reduced, with cathode potentials of 1.5 to 3.0 volts being especially preferred. The applied voltage provided by the power source may vary widely depending upon the IR drop of the reaction medium. The IR drop is preferably minimized to prevent overheating of the reaction cell.

The current density may preferably range from about 0.01 to about 8 amp/in. of cathode surface with 0.1 to l amp/in. being especially preferred. At higher current densities, the selectivity of the reaction decreases; therefore, the current density should be adjusted to give the desired minimization of by-products.

The temperature of the electrolysis reaction may vary widely. The temperatures may be varied to maintain the cell contents as a liquid phase with temperatures from about 0 to about 100 C. or more being preferred and temperatures of about 20 to about 60 C. being especially preferred.

The reduction is usually and most conveniently carried to less than 100% conversion to minimize the over-reduction of the product under the reaction conditions. As a general rule, 70 to of the reduction theoretically required gives the most favorable yields of the desired product with minimum by-product. After the electrolysis, the product may be isolated by any conventional method.

The resultant compounds are useful fungicides for various fungi, such as Staphylococcus aureus, rice blast, Candida albzcans and Rhizopus nigricans. The compounds are also useful herbicides for plants, such as pig weed, crabgrass, yellow foxtail and water plant-milfoil.

SPECIFIC EMBODIMENTS Example 1.Reduction of tetrachloro-Z-trichloromethylpyridine An electrolysis cell was constructed in a 400 ml. beaker by placing a A" deep pool of mercury on the bottom of the beaker to serve as the cathode, a graphite anode and contacts to the electrodes connected to a constant voltage power supply. The cell also contained a saturated calomel electrode for measuring the cathode potential. A magnetic stirring bar was placed on the pool of mercury to agitate the mercury and a propeller stirrer was placed in the solution. The mercury cathode had a surface area of about 6 sq. in. To the cell was added a solution of 20.0 g. of tetrachloro2-trichloromethylpyridine in 150 ml. of 1,2-dimethoxyethane, 125 ml. of methanol and 25 ml. of 30% aqueous H SO The electrolysis was begun at 30 C. and during the course of the electrolysis, the temperature rose to 68 C. The applied voltage was 28 volts for the first 25 min., 23 volts from 25 to 110 min. and 18 volts thereafter until the electrolysis was terminated at 140 min. The cathode potential ranged from 1.8 to 1.3 volts. The current was constant during most of the reaction at 1.4 amps. At the end of the electrolysis, the cell fluid was recovered, stripped of solvent and 17 g. of product was isolated. The product was analyzed by vapor phase chromatography and mass spectroscopy to be 5.5% tetrachloro2trichloromethylpyridine (the starting material), 87.5% was tetrachloro-Z-dichloromethylpyridine, M.P. 68-69 'C., and 7.0% was tetrachloro- 2-monochloromethylpyridine, M.P. 54-57 C. The current efliciency was about 97% Example 2- Reduction of tetrachloro-4-trichloromethylpyridine In the electrolysis cell of Example 1, a solution of 10.0 g. of tetrachloro-4-trichloromethylpyridine in 100 ml. of 1,2-dimethoxyethane, 75 ml. of methanol and 15 ml. of 30% aqueous H SO was electrolyzed. The electrolysis was conducted for 80 minutes, the temperature rose from 28 C. to 67 C., the applied voltage decreased from 30 to 20 volts, the cathode potential decreased from 1.3 to 1.05 volts and the current flow increased from 1.0 to 1.45 amps. After the reaction, the cell fluid was worked up to give 8.5 g. of a product consisting of 94.5% tetrachloro-4-dichloromethylpyridine, M.P. 84-87 C., 2.5% tetrachloro 4 monochloromethylpyridine, M.P. 124- 126 C., and 3.0% 2,3,5,6-tetrachloro-4-methylpyridine.

Example 3.-Reduction of 2-chloro-6-amino-4- trichloromethylpyridine In the cell of Example 1 equipped with a glass cup having a fritted glass bottom around the anode, a solution of 4.0 g. of 2-chloro-6-amino-4-trichloromethylpyridine was electrolyzed for 142 minutes. During the electrolysis, the temperature varied between 32 and 46 C., the applied voltage ranged from 5 to 7 volts, the cathode potential ranged from 1.5 to 2.1 volts and the current ranged from 0.27 to 0.45 amps. Three grams of product were isolated which was identified to be 2-chloro- 6amino-4-dichloromethylpyridine and 5% 2-chloro-6- amino-4-monochloromethylpyridine.

In the same manner as shown by the examples above, the chlorines on the halogenated pyridines may be replaced with fluorine to give, for example, perfluoromethylpyridine and the fluorines are removed from the methyl group without affecting the ring substituted halogens. Also in the same manner, 3-halomethylpyridines are reduced.

Moreover, in the same manner as shown by the examples above, one or two of the aromatic halogens are replaced with CN, NH N(CH N(CH )C6H13, OH, OCH or OC I-I and the compound is electrolyzed to remove halogens from the methyl group.

What is claimed is:

1. An electrolytic process for removing at least one chlorine atom from the chlorinated methyl group of a compound of the formula D Qi-D Xn Ym wherein X is Cl p is an integer of 1-3 n is an integer of 1-4 Y is CN, NR or OR where R is H or an alkyl of 1 to 6 carbons, and

m is an integer of 0-1 which comprises applying to the cathode an electrical potential of about -1 to 3 volts measured against a saturated calomel electrode and a current density of about 0.01 to about 8 amp./in. of cathode surface to a solution of said compound in an organic solvent which solvent also contains a neutral or acidic electrolyte.

2. The process of claim 1 wherein m*=0.

3. The process of claim 1 wherein n+p=7.

4. The process of claim 1 wherein Y is NH and m is 1.

5. The process of claim 1 wherein the chloromethyl group is substituted in the 2 or the 4 position.

References Cited UNITED STATES PATENTS 1,627,881 5/1927 Bellone 20473 3,425,919 2/ 1969 Nagao et al 204-73 FOREIGN PATENTS 762,873 5/ 1921 Canada 204-72 FREDERICK C. EDMUNDSON, Primary Examiner 

